1. Field
The disclosed concept pertains generally to direct current (DC) circuit interrupters and, more particularly, to DC arc fault circuit interrupters. The disclosed concept further pertains to DC arc fault detectors. The disclosed concept also pertains to methods of detecting arcs in DC power circuits.
2. Background Information
When a harmful arc occurs in an electric power system, it produces broadband electrical noise that propagates through the conductors of the power system. Most arc fault detectors (AFDs) work by monitoring the broadband noise in a power circuit and causing a trip if the broadband noise fits expected signal characteristics of an arc.
U.S. Pat. No. 8,089,737 discloses an alternating current (AC) arc fault circuit interrupter (AFCI) including separable contacts, an operating mechanism to open and close the separable contacts, and an arc fault detector to detect an arc fault condition operatively associated with the separable contacts. The arc fault detector includes a tuned current sensor to sense broadband noise of an AC current flowing through the separable contacts, a compression circuit, such as a demodulating logarithmic amplifier, including an input of sensed broadband noise from the tuned current sensor and an output. The compression circuit compresses the dynamic range of the sensed broadband noise. A minimum detector includes an input of the compression circuit output and an output of the minimum value of the minimum detector input. A processor includes a number of inputs and an output. One of the inputs is the minimum value output of the minimum detector. A trip mechanism cooperates with the processor output and the operating mechanism to trip open the separable contacts responsive to the detected arc fault condition.
Solar panels (e.g., without limitation, collections of relatively large photodiodes) are passive devices and typically produce no broadband noise when generating power. In a solar power system, an inverter is used to convert direct current (DC) power generated by a photovoltaic (PV) array into AC power similar to what is distributed by conventional electric utilities. Inverters tend to produce discrete bursts of broadband noise when power devices within the inverter change state. Since a large component of the power dissipated in an inverter is directly proportional to the duration of the inverter switch turn-on and turn-off times, this switching time is usually minimized.
On the other hand, an arc tends to conduct indefinitely once struck. This is particularly true in solar power systems, where the available voltage (which can be several hundred DC volts) is usually much greater than the minimum voltage required to sustain an arc (e.g., about 30 to 70 VDC) and where there are no voltage “zero crossings” as in AC power systems.
Hence, a DC arc fault detector needs to accurately distinguish between relatively short duration noise bursts produced by inverters (the normal case) and continuous, relatively long duration noise produced by arc faults (an extremely rare case).
Another complication is that the noise bursts produced by a particular inverter may be much higher in amplitude than continuous noise that results from an arc fault.
A still further challenge is to correctly distinguish between noise produced by arc faults and spurious signals that may couple into the PV array from radiating sources or leak in from the AC side of the inverter.
There is room for improvement in arc fault detectors for direct current power circuits.
There is also room for improvement in arc fault circuit interrupters for direct current power circuits.
There is still further room for improvement in methods of detecting arcs in direct current power circuits.